In recent years, CDMA mobile communication system has been developed greatly and employed widely, whose most typical examples are CDMA95, CDMA2000 mobile communication systems commonly used currently in commerce. Compared with mobile communication systems with other mechanisms (such as the time division multiple access TDMA system, the frequency division multiple access FDMA system), the CDMA system has advantages in capacity and service quality. However, because users of CDMA system share one single wideband channel, the common channel interference (CCI) among respective users will be resulted, which limits the capacity of the CDMA system. Furthermore, the rapidly increasing number of mobile communication users and continuously increasing demand on quality of mobile telecommunication require the cells to remain high talking quality while having a great capacity. On the respects of lowering the interference of the system and improving the capacity, the method commonly used is to use the technique of sectoring the cell, too many of sectors, however, will bring about too much handoff load to the system, and will generate pilot frequency pollution, therefore, the existing base station equipments commonly employ there or six sectors. In dense urban area, it is sometimes necessary to further improve the capacity and performance of the system on the basis of the sector base station. When in such situation, some other techniques are needed, such as the multi-carrier technique, the smart antenna technique.
The smart antenna technique is a new kind of mobile communication techniques, including the fixed multi-beam system and the adaptive multi-beam system. The multi-beam handoff system is a system which adds a handoff logic to the fixed multi-beam system so as to facilitate the handoffs among the beam systems. In some documents, the fixed multi-beam system and the multi-beam handoff system are called by a joint name as fixed beam system, the same is true in the present invention.
The smart antenna system controls the direction of the beams by using antenna array, and is capable of tracking the changes of the signals. Compared with all direction antennas and sector antennas, the smart antenna system can improve the gains of antennas via narrow beams, therefore can improve the reception quality of signals greatly.
Theoretically speaking, the smart antenna system with N antenna meta can bring the gain of 10 LogN dB in the condition of adding white Gauss (AWGN) channel, so with the same array transmission power, the smart antenna system can increase the cover area of a base station, for example, when the path loss index n=4.5, and 8 meta array is used, the number of the base stations needed to cover this area will be decreased by 60% compared with the number of base stations using the common antenna system. While maintaining the same cover area, the smart antenna system can improve the performance of the base station system, decrease the transmission power of the meta, and increase the cost benefits of the base station.
The adaptive antenna system forms the transmission and reception beams of the user signals by adopting a certain adaptive rule. Though it has great flexibility and has good performance gains, the system is complicated in realization, and is very difficult to be updated to the smart antenna system based on the existing system.
Since the fixed beam system in the smart antenna system can further divide the sectors via the narrow beams, the capacity of the smart antenna system is greatly increased compared to the none smart antenna system with sectors. For example, FIG. 1 is a schematic view of a common CDMA base station system with sectors. In FIG. 2, each sector in FIG. 1 is further divided into multiple beams which share one pilot, so the interference and handoff are reduced, and the capacity of the system is increased.
The fixed beam-forming network can use either the digital beam-forming technique or the analog beam-forming technique.
The smart antenna has incomparable advantages in aspects of eliminating interference, enlarging cover area of cells, decreasing system costs, and improving system capacity.
The fixed beam smart antenna system is relatively suitable to be realized in the existing CDMA systems, such as CDMA95, CDMA2000 and WCDMA systems, while the adaptive beam antenna system is comparatively compatible to be realized in the new CDMA systems, such as TD-SCDMA system. The adaptive beam smart antenna system requires more complicated algorithm and correction technique than the fixed beam smart antenna system does.
When using the fixed beam system in CDMA system, it not only needs to form multiple fixed narrow beams for carrying the service information of users, but also the sector beams covering the whole sector, which is used for transmitting the common information of users, such as the information of the pilot channel, the paging channel and the synchronization channel.
Currently, many realizable schemes have been proposed in this research area.
One scheme is to transmit the common information when all the fixed beams are transmitting the traffic channels. However, the pilot channel of each beam, whose vector sum is the strength of the pilot channel of any point in the space, is correlated. When adding the vectors of each beam together to form the pilot channel, the overlays between beams will result in bad addition in some area, and the so-called coverage blind area, where when the mobile stations enter, the talking quality will be decreased greatly, and the call will be even disconnected.
Another scheme is that besides the radio frequency channels and the antennas for generating the fixed narrow beams, another one or group of radio frequency channels and antennas are added to generate beams covering the whole sector for transmitting the common information (such as information of the pilot channel, the paging channel and the synchronization channel). However, this method needs additional high power radio frequency elements and transmission antennas, which will increase the realization cost of the system. In addition, because the pilot channels and the traffic channels pass different radio frequency links in this method, a complicated correction technique will be needed to overcome the problem of time and phrase offsets of the pilot channels and the traffic channels due to the variations of time and temperature. The U.S. Pat. No. 6,094,165 disclosed such a scheme as generating a fixed beam covering the sector by using an additional array antenna.
There is still another scheme in which a certain frequency offset exists between each fixed beam, so as to reduce the bad addition between the beams when synthesizing pilot channels. The related content of it can refer to the US patent No. US2002/0072393, in which the proposed frequency offset is 30˜120 Hz, that is, if each sector has 3 beams as commonly used, the carrier offset of this part will reach ±30˜120 Hz,I S-97 (the minimum testing criteria for the performance of base station in CDMA95 and CDMA2000) prescribes that the TX frequency tolerance of the base station should <±0.05 ppm, therefore, if the frequency of the base station is 800 MHz, merely the frequency offset of this part will be close to or greatly excess the frequency tolerance of the base station transmission, and therefore the offset of the reception frequency will decrease the demodulation function of the mobile station.